We investigate the ferroelectric domain architecture and its operando response to an external electric field in BaTiO3-based electro-optic heterostructures integrated on silicon. By noninvasive optical second-harmonic generation, we identify the preexistence of in-plane (a-) domains dispersed within a predominantly out-of-plane-(c-) oriented matrix. Monitoring the poling behavior of the respective domain populations, we show that the spontaneous polarization of these a-domains lack a predominant orientation in the pristine state, yet can be selectively aligned with an in-plane electric field, leaving the c-domain population intact. Hence, domain reorientation of a ferroelastic c-to-a type was directly excluded. Such independent electrical control of ferroelectric a-domains in a c-oriented BaTiO3 film on silicon is a valuable platform for engineering multidirectional electro-optic functionality in integrated photonic devices.